Hepatitis C virus (HCV) is a leading cause of chronic hepatitis and hepatocellular carcinoma worldwide and infects more than 1% of the world population. Successful vaccine development is pivotal in controlling this global health problem. A system for efficient assembly of HCV structural proteins into HCV-like particles (HCV-LPs) in insect cells has been developed in our laboratory. These noninfectious HCV-like particles have similar morphologic, serologic and biophysical properties as the putative virions isolated from HCV infected humans. In contrast to recombinant subunit vaccines, the viral proteins of HCV-like particles may be presented in a native, virion-like conformation and may therefore be superior in eliciting a protective humoral and cellular immune response. The humoral and cellular immunogenicity of the HCV-LP had previously been demonstrated in the mouse model. [unreadable] [unreadable] We tested the HCV-LP and the adjuvants in a nonhuman primate model (baboon) and demonstrated induction of robust and broad humoral and cellular immune responses that were marginally enhanced by the AS01 adjuvants. The overall HCV-specific immune responses were robust and long-lasting (>8 months). We have begun the immunization and challenge experiment in chimpanzees. Chimpanzees, two in each group, were immunized with HCV-LP or HCV-LP + adjuvant ASO1B. After four immunizations over an eight-month period, all animals developed strong HCV-specific cellular immune response including IFN-gamma+ and IL-2+, CD4+ and CD8+ T-cell and proliferative lymphocyte responses against core, E1 and E2. The immunogenicity of HCV-LP was not enhanced by the adjuvant. The chimpanzees in both groups were challenged with 100 CID50 of HCV CG1B inoculum. Upon challenge with HCV, one chimpanzee developed transient viremia with low HCV RNA titers (~10E4 copies/ml) in the third and fourth weeks post-challenge. The three other chimpanzees became infected with higher levels of viremia (up to 10E5 copies/ml) but their viral levels became unquantifiable (< 1000 copies/ml) 10 weeks post-challenge. After HCV challenge, all four chimpanzees demonstrated a significant increase in peripheral IFN-gamma+ T-cell and proliferative responses as well as the presence of intrahepatic T-cell response against the HCV structural proteins. T-cell responses against various nonstructural proteins also quickly became detectable. These T-cell responses coincided with the fall in HCV RNA level. Previously, three other naive chimpanzees were challenged with the same HCV inoculum, and two developed persistent infection with viremia in the range of 10E5-6 copies/ml. Our results suggest that HCV-LP immunization induces strong HCV-specific cellular immune responses and confers partial protection against HCV challenge in the chimpanzee model. In an effort to improve and broaden the immunogenicity of HCV-LP, we are engineering T cell epitopes from the nonstructural genes into the HCV-LP. In addition, we are combining The HCV-LP approach with other modalities of immunization, such as plasmid DNA, in a prime-boost regimen.